1. Field of the Invention
The present invention relates to an optical transmission device and a light-receiving module which can be used with a waveguide or the like for transmitting lights.
2. Related Art of the Invention
Examples of a waveguide which transmits lights include a channel waveguide which provides a refractive index difference or a reflection portion in the nonparallel direction, e.g., the perpendicular direction, to an optical axis to transmit the lights by repetition of total reflection or partial reflection and a bulk waveguide which provides a plurality of lenses or mirrors on the optical axis to transmit the lights by performing beam conversion. Note herein that the case where the lights are coupled by a single lens or mirror is classified as the bulk waveguide for convenience.
In addition, researches on an optical waveguiding using a photonic band of a photonic crystal of which a refractive index is modulated in a cycle of an optical wavelength level, which applies a phenomenon impossible to achieve with conventional materials, are also active recently.
The channel waveguide using the photonic crystal includes that a periodic defect portion in a channel form that disturbs the cycle is fabricated in a refractive index modulation structure with a uniform cycle that provides a photonic band gap to a light source wavelength to guide the light only to the periodic defect portion in the channel form (for example, see Japanese Patent Laid-Open No. 11-218707).
FIG. 12 illustrates a configuration diagram schematically showing a photonic crystal waveguide disclosed in the Japanese Patent Laid-Open No. 11-218707.
A photonic crystal waveguide 100 is provided with a slab optical waveguide 102 formed on a substrate 101 made of silicon. In a core layer of the slab optical waveguide 102, refractive index changing regions 108 in a cylindrical form having the larger refractive index than that of the core layer surrounding them are arranged in the form of a regular triangle lattice, to constitute a photonic crystal 109. By creating the periodic defect portion, which is not provided with the refractive index changing region 108, in the photonic crystal 109 having the refractive index modulation structure with the uniform cycle, an optical waveguiding region 105 is formed.
Among input light 106 input from an input optical fiber 103 connected to the photonic crystal waveguide 100, only the light with the wavelength which satisfies the Bragg conditions by the refractive index changing region 108 is confined in the optical waveguiding region 105 to be propagated, and is transmitted to an output optical fiber 104 as output light 107.
However, in such a photonic crystal waveguide 100, an incident beam diameter, which allows the beam to be coupled at an incident side in a similar manner to that of the conventional channel waveguide, is limited and a channel width is made to 1 μm or less within a defective width of one to two cycles, so that it is difficult to be coupled as compared with the conventional general waveguide.
In other words, while the photonic crystal waveguide 100 enables optical transmission to a distance at high speed once it is coupled with the optical fiber or the like and the coupling at the light-receiving side is not affected by the coupling at the incident side, it cannot be coupled with the beam with the diameter of 100 μm or more at the incident side due to a core cross-section thereof for high-speed transmission being small.
Meanwhile, it is known that by using the photonic crystal, a negative refraction angle can be achieved because of a crystal structure thereof (for example, see Japanese Patent Laid-Open No. 2005-4225). An optical coupling device is disclosed wherein the above described problem in difficulty of the coupling with the incident beam with the large diameter is solved utilizing an optical property of the negative refraction angle of the photonic crystal (for example, see Japanese Patent Laid-Open No. 2004-133040).
FIG. 13(A) illustrates a schematic diagram showing a refraction phenomenon in a general material, while FIG. 13(B) illustrates a schematic diagram showing the refraction phenomenon in the photonic crystal when the optical beam enters from the air.
The device, as represented by a lens or a prism, which controls the propagation direction of the optical beam utilizes the so-called refraction phenomenon where the propagation direction of the beam is changed in accordance with a difference between the refractive index of external environment (generally, the air) and that of a device material. In this case, a degree of the refraction angle with respect to the incident angle is determined by the ratio of the refractive indexes of the materials on both sides of an interface (Snell's law). For example, as illustrated in FIG. 13(A), if a medium 111 (approximately 1.5 for a general glass) is larger than a medium 110 (1 for the air) in terms of the refractive index, the refraction angle is then smaller than the incident angle. That is, the refraction angle has the same sign with that of the incident angle.
In contrast, when the photonic crystal is used, the crystal structure thereof enables to achieve the refraction angle which is impossible to achieve by a general optical system. As illustrated in FIG. 13(B), the beam entered from the air with the refractive index of 1 into a photonic crystal 112 can be refracted to the same side with the incident beam, i.e., at the negative refraction angle. By controlling the crystal structure of the photonic crystal 112 using structural parameters, such a negative refraction angle can be achieved.
FIG. 14 is a schematic diagram showing a trail of the light in the optical coupling device of Japanese Patent Laid-Open No. 2004-133040, which constitutes the bulk waveguide utilizing the optical property of the photonic crystal, i.e., the negative refraction angle.
A photonic crystal 120 constituting this optical coupling device is the photonic crystal having the band with a negative inclination with respect to the light source wavelength due to a return, which propagates the light in the direction where the refraction angle is negative with respect to the incident beam.
The light entered from the light source at a point A into the photonic crystal 120 is refracted in the negative refraction angle direction to be propagated within the photonic crystal 120, and is also refracted in the negative refraction angle direction to be propagated when going out therefrom. As illustrated in FIG. 14, by adjusting the refraction angle of the photonic crystal 120 so that the entered refracted beams intersect within the photonic crystal 120, the light emitted from the light source at the point A can be condensed at a point B in the air. The light from the light source at the point A is coupled to an optical fiber 121 by arranging an end face of the optical fiber 121 at the condensing point B.